Replacing a transformer
 

There's a related thread to this in the tech forum but I figured there are more radio guys here.
My type 80 rectifier will no longer light on my current project. I cannot detect any heater current (5v) at the socket. This transformer is not original to the radio and had leaked a bunch of wax prior to when I got it. Below is part of the diagram for the radio and a diagram for a new transformer on eBay. Would the new one be a suitable replacement? Current voltage now to the rectifier is 380.

[IMG][ http://img.photobucket.com/albums/v107/schanz/_12.jpg /IMG]

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That transformer would work. Your 6V filament tubes draw 2.2A and the 80 draws 2A from the 5V winding. Your plate current adds up to 63mA so this transformer is way capable of handling that.

Have you determined that the 5V winding on your transformer is no good?

I've tested it several times and always get .001 to 0v on two legs. The other two legs read 378v. I haven't changed any of the transformer wiring and this radio was working nicely after I replaced all the caps. BTW, I do find 6.5v on the other winding.

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Measure (with power and the 80 removed) the resistance of the 5V winding. It should be very low, less than 1 Ohm. If not, trace the connections and look for a broken or poorly soldered connection.

wire to wire reads O.L. which I believe is infinite. The high side reads .390K. I checked at the solder joints as well. Wire doesn't feel broken. I could expose some bare wire near the transformer to make sure.

Hello,

Please bear in mind that old transformers usually did not use stranded wire as lead exit on their LV windings, but a lenght of tubing over the enamelled copper wire (at least, this is how old transformers in Italy were done) was the usual arrangement. What usually is the problem is the enamel is not scraped very well to take solder and to make a decent joint. Please, before condemmning the old PT, try to scrape the ends of the 80's filament winding to bare and shining copper, then retest.

I would be really surprised if this winding gave up the ghosts, as it employs few turns of a relatively heavy gauge magnet wire

Cheers

Giulio Maiocco

I cut the wires loose, stripped back some insulation and retested ohms. Still nothing between the wires (0.L.). Measuring voltage to ground, I got 5V on one and 2.9 on the other.

Hello again,

Everything is possible, but what you have does not make sense to me. The magnet wire used to wind the 5V is thick, about a millimeter in diameter, so it's almost impossible to open it up. Is the wire exiting from the transformer a stranded wire? If so, there is a splice under the transformer's paper insulation that may be bad. If this is the scenario, cut back carefully the external layer of insulating paper on the power transformer and, after testing for continuity where the magnet wire joins the stranded wire, resolder the joints and check again.

The voltage test between the winding and ground does mean nothing, as this 5V winding must float with respect to ground to the full B+ voltage. So you should get 5V between the 2 wires composing the winding, voltages to ground are almost surely stray voltages.

Cheers

Giulio Maiocco

The scabby way to fix this would be to leave everything as-is, and solder a pair of 1N4007's and a dropping resistor in place of the 80. The silicon diodes won't need filamant voltage so the open winding becomes not needed. You can leave the 80 in place just for looks. You can figure out the resistor value by trial and error, start with say 200 ohms and work your way down.. a few wirewound 10W resistors would be a nice starting point. An 80 is more or less instant start anyway since it's directly heated. This is the $5 15 minute repair I'd go for, rather than all the nonsense involved in changing the whole power transformer.

Thanks for your time Giulio. I'll try to be clear. I cut the wires where they were soldered at the socket. They are solid strand with a copper coating. Does it mean anything that a bunch of wax-like goo ran out of the transformer some time in the past?

Maxhifi, your solution is tempting. I may need some more details.

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http://archive.siliconchip.com.au/cm...1/article.html

scroll down to "rectifier replacements"

You will soon get the idea after reading.

So to bypass the 80, I would need to know my target volts and amp draw, correct? Looks like 240 v at the other tubes, is that what I'm shooting for?

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You need at least 2 seriesed 1N4007 for each leg of the HT winding. Current handling of the 1N4007 is more than ample, but the problem is the reverse voltage you'll encounter in this application, so my suggestion of using 2 1N4007 in series.

Anyway, solid state diodes in vintage radios :eek: Might be ok as a test, but I would never ever consider this as a definitive solution, unless the rectifier is unobtanium, and even then I would think of an alternative tube to swap.

This is a very interesting fault with this 5V winding, too bad you are not in Italy, otherwise I would fix this for you in no time flat. The melted wax indicates the transformer ran warm in it's life, probably not helped by bad paper and 'lytic caps leaking like sieves!

Cheers

Giulio

I appreciate everyone's help on this but I went ahead and ordered the transformer. Getting more $ in this radio than what is prudent but this was my first antique radio purchase and I really want it working. It's been sitting on the shelf far too long.

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Installed last night. Works like a charm. May need to paint it black though.

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I'm glad that this thread was made, and even more glad concerning the outcome of Schanz's radio. Reason being on the former is that I have an old Delco R-1125 with a #80 rectifier that has been giving me fits. Basically the B+ is really low and I can't find a reason why so far. The transformer gets really hot after about a half hour.

I replaced the filters and a number of the non polarized caps. I haven't done the things mentioned here as far as having all leads open and checking the resistance of windings and shorts between windings or to ground. The main reason is that the wiring is so hard that I feel that it would be destroyed by moving it much. The insulation under chassis is good as it stands. I popped the covers on the trans. and it wasn't very pretty, but didn't seem to be smoked.

Like Shantz, I feel that opting for a replacement trans. would save a lot of time and aggravation. I don't know where I would find one, but if anyone does please let me know. Thanks!

Could be that you need to replace all the caps first. I don't know that the smaller down stream caps can heat your transformer like that but I'm no expert by any means.

I put mine all together last night finally but when I turned it on I heard a small electrical arc. Uh oh. I found one my 22uf electrolytic caps had come loose from the ground. Repaired the solder but now the sound is gone. Replaced that cap but that didn't fix it. Voltages at the rectifier are fine. All tubes light up but my b+ voltage is missing farther down the circuit if that makes sense. Haven't had time to investigate much yet. Frustrating.

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Make sure that both electrolytics don't go to ground. The C-25 negative goes to the center tap of the transformer and speaker field as the choke and forms the bias supply for the 42 tube. It could be that the R-13, 235 ohm resistor opened. As a result of this resistor opening, there won't be any B+ to the rest of the set, not even a hum. They usually used those flexible type resistors. Replace with at least a two watt wire wound resistor.

Thanks. I'll check r13. It's one of the few original ones left. C26 is the only one that goes to ground and is the one that popped loose. Quick question though. I can measure 360v across C25 (pos to neg lead) but nothing across C26. It's that proper?

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Impossible to tell without seeing the schematic :)

Right.

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The field coil and the R-13, is the only return to chassis ground. Did you check the 235 0hm and the field coil. It should read around 1050 ohms.
BTW, the speaker should be plugged in, to complete the B+,B- circuit.

I get about 1100 ohms on two pins of the speaker plug while disconnected so maybe we're ok there. R13 seems to check out ok also. It's an armored wire wound thing attached to the chassis. Voltage before the field coil was about 230 vdc. It was about 30 vdc after. Still getting approx 30 vdc after r14 but nothing after r15. This was done really quickly on my way out the door this morning. I'll replace both r14 and 15 tonight and measure again.

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So far, everything in the bias supply, seems to be in order. I wouldn't worry about the R14, R15, at this time!
I can't see why, there is no B+ at the C26, measured to ground. :sigh:
The voltage chart is on page three of the Rider's.
The B+ measured from the + side of C-26, should be around 240 volts, DC, to chassis ground. :yes:

Hmmm...
Could C25 be bad? It is a replacement that was done some time in the past by someone else.
I believe I will go through this thoroughly when I have more time and write voltages on the schematic.

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It would have to shorted big-time. You voltages on the bias source seem to be right.
Keep us informed! :thmbsp:

Took some measurements and wrote down some voltages.
The speaker makes a little buzzing and the back of it gets hot. Never really felt the speaker coil before.


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Careful here, Schanz, if the voltages you reported are right, you are pulling way too much current from the 80 rectifier and power transformer. The field coil shoud get warm over a hour or so, but you say the field coil on the back of the speaker becomes hot, this confirms overload.

Just to start, with the radio unplugged, what's the resistance reading between the 80 filament (doesn't really matter which one of the two) and the chassis? With the symptoms you describe, I would expect a very low resistance, if this is the case, this is not right. Check C26, C28, the primary of the OPT and the 42 tube for shorts to chassis, or, failing these checks, the wiring carrying the B+ voltages around in the radio. Avoid as much as possible to plug in and turn on the radio, as this may cost you another power transformer / 80 tube / field coil! Oh, BTW, I hope that 6VAC on the 80's filament is a mistypo, as this would ruin the tube in a short time

Just let me know if I can help you further

Giulio

Yes Giulio, something was definitely wrong. Your comments got me thinking so before leaving for work this morning I checked everything carefully with a flashlight. I found my transformer had shifted and my make-shift mounting bracket had shorted a leg of the 80 to the chassis. That explains the missing voltage. Amazingly, the radio still works. The transformer will get a permanent mounting solution and all bare leads will get insulated.

And yes, that 6v on the 80 was a mistake. It is actually 5.25.

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:thmbsp: I think you found your problem , once you fix your PT in a secure and permanent way, I bet you'll find your 240V or so volts between the 80's filament and chassis and, important thing, you'll find the negative voltage generated across the 235 ohm will be quite less than your -31.6V, it should be roughly -17V.

With that dead short, no wonder you heard a pop/arc!

Ok also about the 5.25V on the rectifier's filament, much much better!:yes:

Best regards

Giulio

Need to add the bolts to the chassis and make a loop antennae.


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A radio of this vintage was made to use a long wire antenna, not a loop. For local use, ten or fifteen feet of insulated wire run around the baseboard, or under the edge of the rug, or up behind the curtain and hooked to the curtain rod, connected to the antenna terminal, will work.

The reason that built-in loop antennas work on later radios is that they were designed to be substituted for the small antenna coil that was used in earlier sets. Since earlier sets have the antenna coils built in, the tuning capacitor already has its coil and any other coil added to it just upsets the capacitive/inductive pair.

It is possible, however, to build a large tuned loop separate from the radio and couple it to the radio's antenna terminal with a one-turn sensing loop wound alongside the tuned loop.

Some long wire sets will work decently with a loop connected only to the antenna terminal, not as good as a proper long wire, but if you can't run an antenna wire it's the next best thing.